The aim of this proposal is to develop genetically-engineered live, attenuated parenteral vaccine sagainst Dengue types 1-4 and West Nile (WN) encephalitis. The technology (ChimeriVax/TM) involves creation of chimeric viruses in which the envelope genes of the vaccine target virus dengue or WN) containing all the epitopes for neutralizing antibodies as well as protective CTL determinants are substituted for the corresponding genes of yellow fever (YF) 17D vaccine virus. The resulting chimera is a live vaccine, which, like YF 17D elicits strong humoral and cellular immunity but is antigenically specific for the heterologous virus. Wild type dengue strains will be used as gene donors, and the chimeric YF/dengue vaccine candidates will have an acceptable safety profile in animal models. In the case of the YF/WN chimera, multiple independently attenuating mutations will be introduced in the E gene of the chimeric YF/WN virus, that render the virus safer (less neurovirulent) than YF 17D. Safety, immunogenicity and protective activity of all vaccine candidates will be established in non-human primates. A monovalent YF/dengue-2 vaccine candidate will first be prepared at clinical grade, and a proof-of-principle Phase 1 trial performed under IND to demonstrate validity of the approach in humans. Optimized YF/dengue 1,3 and 4 vaccine candidates will be developed, tested in monkeys, and manufactured under GMP. A tetravalent YF/dengue vaccine will be formulated, dose composition determined by factorial studies in monkeys, and tested in a Phase 1 clinical trial. The YF/WN vaccine will be brought through development and tested in a Phase 1 trial. The potential of the YF/WN as a veterinary product to protect horses will also be investigated.